Control device for double-acting fluid pressure cylinders



ay 16, 1967 s. o. E. SJBERG 3,319,530

CONTROL DEVICE FOR DOUBLE-ACTING FLUID PRESSURE CYLINDERS Filed May ll, 1965 United States Patent O 3,319,530 CGNTROL DEVICE FOR DOUBLE-ACTING FLUID PRESSURE CYLINDERS Sten Olof Evald Sjberg, Sunzlbyberg, Sweden, assigner to Indnstriaktiebolaget Skomab, Nasby Park, Sweden Filed May 11, 1965, Ser. No. 454,940 Claims priority, application Sweden, May 14, 1964, 5,899/64 Claims. (Cl. 91-275) This invention relates to -control devices for doubleacting fluid pressure cylinders and particularly to a device for limparting a reciprocating motion about a displaceable centre to the piston and piston rod in a double-acting Huid pressure cylinder controlled by a solenoid-operated valve, e.g. the working cylinder in a hydraulic bending or anging machine.

Hitherto the working cylinder in hydraulic bending or flanging machines has been controlled through a hand or foot-operated valve arranged to pass the pressure uid to one or the other side of the piston. The number of strokes per minute of the piston and piston rod then depends on the rapidity with which the valve is actuated. To obtain a satisfactory bending or anging on a template the number of strokes per minute must be quite high, and, if not completely impossible, it is not possible to obtain the required number of strokes per minute without a considerable effort.

The invention has for its object to provide a control device with which it is possible to impart to the piston and piston rod in a double-acting fluid pressure cylinder a reciprocating motion about a continuously displaceable centre with a sui-ciently high number of strokes per minute.

According to the invention there is provided a control device for imparting a reciprocating motion about an adjustable centre to the piston and piston rod in a doubleacting fluid pressure cylinder controlled through a valve Operated by two solenoids, comprising a change-over switch arranged to pass operating current from a power supply to either one of the solenoids according to its momentary state and to be switched over by a bistable levelsensing device of the kind wherein a continuously variable input voltage corresponds to either one of two constant output voltage levels and wherein, with an increasing input voltage, transition from one level to the other occurs for an input voltage higher than the input voltage for which transition from said other level to said one level occurs with a decreasing input voltage, the input voltage being controlled by the resulting impedance of two independently variable impedances, of which impedances one is positively varied from a maximum to a minimum value as the piston and piston rod move from a rst dead centre to a second, and vice versa, while the other is manually adjustable at will.

In a preferred embodiment of the invention the impedances are provided by variable capacitors of which the capacitor associated with the said positively varied impedance is mechanically coupled to the piston rod while the other capacitor is mechanically coupled to a pedal or a hand grip. Conveniently the change-over switch is bistable multivibrator While the bistable level-sensing device is a Schmitt-trigger.

One embodiment of the invention will now be described with reference to the accompanying drawing which diagrammatically illustrates electronic and hydraulic cornponents in a hydraulic bending or anging machine.

The machine is provided with a double-acting hydraulic working cylinder 1 having a piston 2 and piston rod 3. The piston rod carries a tool 4 for reciprocating with respect to and coacting with an anvil 5. The reciprocating ICC motion is controlled by a four-way solenoid-operated valve 6, which has three positions and is spring centered. When the valve 6 is uncentered, i.e. in either end position, pressure o-il from a hydraulic power unit 7 is passed to one or the other of the cylinder compartments above or below the piston 2, and when the valve is centered, i.e. is in its intermediate position, the connections to the cylinder 1 are blocked so that the piston is hydraulically locked.

The two solenoids 8, 9 of the valve 6 are energized from an electric power supply 10 through a change-over switch in the form of a bistable multivibrator 11 and a foot-operated switch 12 inserted into the supply circuit of the solenoid S. Depending on the momentary state of the bistable multivibrator 1.1 one or the other of the two solenoids is energized (switch 12 is assumed to be closed), and thus the pressure oil is passed to either the upper cylinder compartment or the lower cylinder compartment. The bistable multivibrator 11 is controlled by the output voltage of a Schmitt-trigger 1'3 the input of which is connected to a voltage source 1-6 through two variable capacitors 14, 15 connected in parallel. The voltage source 16 is, in the embodiment disclosed, an oscillator of a fairly high frequency; this is to keep the capacitances at a reasonable value. Of course, it is possible to use other voltage sources. The capacitor 14 is mechanically coupled to the piston rod 3 in such a way that its capacitance is continuously increased as the piston and piston rod travel downward, and vice versa. The capacitor 15 is mechanically coupled to a pedal 17 in such a way that its capacitance is continuously decreased when the pedal is depressed and is increased when the pedal is released.

The operation of the device is as follows: At a certain moment the output of the Schmitt-trigger 13, which operates as an on-oif switch or a relay having hysteresis, is dead or switched 0E, i.e. there is no or only a small voltage applied between the output terminals, whereby, in this embodiment, the solenoid 8 is energized from the power supply 10 through the bistable multivibrator 11. The solenoid 8 holds the valve 6 in the position in which oil is passed from the power unit 7 t-o the upper cylinder compartment and therefore the piston 2 and piston rod 3 are forced downwards. As the piston and piston rod travel downwards the capacitance of the capacitor 14 is increased and therefore the input voltage of the Schmitttrigger 13 is increased. When the capacitance of the capacitor 14 and therefore the input voltage of the Schmitt-trigger 13 is increased suiciently, the Schmitttrigger is switched on to a live output, and hence the bistable multivibrator 11 switches over the current from the power supply 10 from the solenoid 8 to the solenoid 9. Then, the solenoid 9 sets the valve 6 in the position in which oil from the power unit 7 is passed to the lower cylinder compartment. 'Ihe piston and piston rod are then forced upwards. This means that the capacitance of the capacitor 14 and therefore the input voltage of the Schmitt-trigger 13 decrease until the Schmitt-trigger is again switched off and resets the bistable multivibrator to the rst mentioned stable state. When this occurs the current from the power supply 10 is switched back to the solenoid 8 so that the valve 6 is again brought to pass the oil to the upper cylinder compartment.

Thus the piston 2 and piston rod 3 reciprocate about a given centre. The length of the stroke and therefore the number of strokes per minute are determined by the input voltages at which the Schmitt-trigger is switched from alive to dead output and from dead to alive output. These voltages are, in well-known manner, adjustable on the Schmitt-trigger, for example, within a range corresponding to a range of 25 to 400 strokes per minute.

The centre of the reciprocating motion of the piston and piston rod may be displaced by changing the capacitance of the capacitor 15 by means of the pedal 17. If

the pedal is depressed the capacitance is decreased and therefore also the resulting or combined capacitance range of the two variable capacitors is shifted downwards. However, since the resulting capacitance limits, for which the Schmitt-trigger is switched on and off, are unchanged the capacitance range of capacitor 14 must be shifted upwards correspondingly so that the sum of the two capacitances Vremains unchanged 'at least for the dead point positions o-f the piston and pist-on rod. Therefore, the two deadpoints of the piston motion will be shifted downwards a distance corresponding to the necessary shifting upwards of the limits between which the capacitan-ce of the capacitor 14 is varied, and thus the center of the reciprocating motion will be shifted downwards the same distance.

The piston and its piston rod may be arrested in a desired position in their downward stroke by means of the foot-operated switch 12. If the vsolenoid 8 is being energized and the foot-operated switch is opened, energization is interrupted and the valve 6 is put to its intermediate position under the action of its centering springs to block the two connections to the cylinder 1 so that the piston and piston rod are hydraulically locked.

It is to be noted that vari-ous modifications are possible within the inventive concept. For example, the two capacitors 14, 15 may be replaced lby variable inductors or resistors. Also the Schmitt-trigger may be replaced by a device having similar characteristics, for example, a suitable electromagnetic relay.

I claim: Y

1. A control device for imparting a reciprocating motion about an adjustable centre to a piston and piston rod in a double-acting uid pressure cylinder controlled through a Valve operated by two solenoids, comprising a changeover switch arranged to pass operating current from a power supply to either one of the solenoids according to its momentary state and to be switched over by a bistable level-sensing device of the kind wherein a `continuously variable input rvoltage corresponds to either one of two constant `output voltage levels and wherein, with an increasing input voltage, transition from one level to the other occurs for an input voltage higher than the input voltage for which transition from said other level to said one level occurs with a decreasing input voltage, the

input voltage being controlled by the resulting impedance y of two independently variable impedances, of which impedances one is positively varied from a maximum to a minimum value as the piston and piston r-od move from a rst dead centre to a second, and vice versa, while the other is manually adjustable at will.

2. A control device as claimed in claim 1 wherein said one impedance is provided by a variable capacitor which 4. A control device as claimed in claim 1 wherein said change-over switch is a bistable multivibrator.

5. A control device as claimed in claim 1 wherein said bistable level-sensing device is a Schmitt-trigger.

References Cited by the Examiner UNITED STATES PATENTS 2,529,777 1 l/ 1950 McLinnis 91-275 2,735,405 2/1956 Hipple 91-275 2,803,110 8/ 1957 Chittenden 91-275 2,851,014 9/1958 Healy 91-275 MARTIN P. SCHWADRON, Primary Examiner.

" PAUL n. MAsLoUsKY, Examiner. 

1. A CONTROL DEVICE FOR IMPARTING A RECIPROCATING MOTION ABOUT AN ADJUSTABLE CENTRE TO A PISTON AND PISTON ROD IN A DOUBLE-ACTING FLUID PRESSURE CYLINDER CONTROLLED THROUGH A VALVE OPERATED BY TWO SOLENOIDS, COMPRISING A CHANGEOVER SWITCH ARRANGED TO PASS OPERATING CURRENT FROM A POWER SUPPLY TO EITHER ONE OF THE SOLENOIDS ACCORDING TO ITS MOMENTARY STATE AND TO BE SWITCHED OVER BY A BISTABLE LEVEL-SENSING DEVICE OF THE KIND WHEREIN A CONTINUOUSLY VARIABLE INPUT VOLTAGE CORRESPONDS TO EITHER ONE OF TWO CONSTANT OUTPUT VOLTAGE LEVELS AND WHEREIN, WITH AN INCREASING INPUT VOLTAGE, TRANSITION FROM ONE LEVEL TO THE OTHER OCCURS FOR AN INPUT VOLTAGE HIGHER THAN THE INPUT VOLTAGE FOR WHICH TRANSITION FROM SAID OTHER LEVEL TO SAID ONE LEVEL OCCURS WITH A DECREASING INPUT VOLTAGE, THE INPUT VOLTAGE BEING CONTROLLED BY THE RESULTING IMPEDANCE OF TWO INDEPENDENTLY VARIABLE IMPEDANCES, OF WHICH IMPENDANCES ONE IS POSITIVELY VARIED FROM A MAXIMUM TO A MINIMUM VALUE AS THE PISTON AND PISTON ROD MOVE FROM A FIRST DEAD CENTRE TO A SECOND, AND VICE VERSA, WHILE THE OTHER IS MANUALLY ADJUSTABLE AT WILL. 